Reading deeply nested key in ES6 object

safeRead.js

/* safely read deeply nested keys
 *
 * obj - the object to read from
 * s   - a string representation of the keys like 'my.deeply.nested.key'
 * arguments - individual keys
 *
 * Usage:
 * 1) safeRead(someObj, 'my', 'deeply', 'nested', 'key')
 * 2) safeRead(someObj, 'my.deeply.nested.key')
 *
 * common usage safeRead(someObj, 'my.deeply.nested.key') || 'defaultValue'
 *
 * // significantly modified from https://stackoverflow.com/a/2631521/1760776
*/
function safeRead(obj, s){
    let keys;
    if (arguments.length > 2) {
      let args = new Array(...arguments); // because arguments is a pretend Array
      keys = args.slice(1);
    } else {
      // s is a string like 'my.deeply.nested.key'
      keys = s.split('.')
    }
    while(obj && keys.length) obj= obj[keys.shift()];
    return obj;
}
export default safeRead;

safeRead.spec.js

// Jest spec
import safeRead from 'libraries/safeRead.js';


describe('safeRead', () => {
  let nestedObj = {a: {b: {c: {d1:1, d2: 2000}}}};

  describe('with a single string arg', () => {
    it('returns the correct value',  () => {
      expect(safeRead(nestedObj, 'a.b.c.d2')).toEqual(2000)
    });

    it('returns undefined if the nested key is not present', () => {
      expect(safeRead(nestedObj, 'a.b.c.alskdjfj')).toEqual(undefined)
    })

    it('returns undefined if the first key is not present', () => {
      expect(safeRead(nestedObj, 'lkadjsf')).toEqual(undefined);
    })
  });

  describe('with multiple string args', ()=> {
    it('it returns the correct value', () => {
      expect(safeRead(nestedObj, 'a', 'b', 'c', 'd2')).toEqual(2000)
    });

    it('returns undefined if the nested key is not present', () => {
      expect(safeRead(nestedObj, 'a', 'b', 'copus', 'fizzle')).toEqual(undefined)
    })
  })

  it('returns undefined if the object itself is undefined', () => {
    let obj = undefined;
    expect(safeRead(obj, 'asdf')).toEqual(undefined)
  })
});